I want to know about selection procedure for Indian Air Force flying branch after B.Tech (Mechanical engineering)?

Yes As a Graduate/ Engineer, you are eligible to apply for Flying branch through the Air Force Academy. You can apply through AFCAT for Short Service Commission (SSC) in the Flying Branch.

Educational Qualifications
You have to pass Graduation in any discipline from a recognized University and you must obtained a minimum 60% marks in aggregate in all papers put together and have Physics and Mathematics at 10+2 level
OR
You must pass BE / B Tech (Four Year Course) from a recognized university with minimum 60% marks in aggregate in all papers put together.

Nationality - Indian
Marital Status – Single
Gender - Men and Women.
Age - 19 to 23 years (at the time of commencement of course).

Medical Standards for the Flying Branch (Additional)
Minimum Height - 162.5 cms
Leg Length - Minimum- 99 cms.Maximum- 120 cms
Thigh Length - Maximum- 64 cms
Sitting Height -Minimum- 81.5 cms. Maximum- 96 cms

Vision
6/6 in one eye and 6/9 in other,
correctable to 6/6 only for hypermetropia. )
Manifest Hypermetropia must not exceed +2.0D Sph, Myopia – Nil and Astigmatism +0.75 D Cyl (within +2.0 D – Max)
Near Vision N-5 each eye.
Color Vision CP-1(ML T).